Let's do some sleep math. You lost two hours of sleep every night last week because of a big project due on Friday. On Saturday and Sunday, you slept in, getting four extra hours. On Monday morning, you were feeling so bright-eyed, and you only had one cup of coffee, instead of your usual two. But don't be cheated by your energy: You're still carrying around a heavy load of sleepiness, or what experts call " sleep debt "win this case something like six hours, almost a full night's sleep.
Sleep debt is the difference between the amount of sleep you should be getting and the amount you actually get. It's a deficit (赤字)that grows every time we skim some extra minutes off our nightly sleep. "People accumulate sleep debt gradually without being noticed," says psychiatrist William C. Dement, founder of the Stanford University Sleep Research Centre. Studies show that such short-term lack of sleep
leads to a foggy brain, worsened vision, and trouble remembering. Long-term effects include obesity, insulin(胰岛素) resistance, and heart disease. A survey by the National Sleep Foundation reports that we're losing one hour of sleep each night —more than two full weeks of sleep each year.
The good news is that, like all debt, with some work, sleep debt can be repaid. Adding an extra hour or two of sleep a night is the way to catch up. For the long-term lack of sleep, take it easy for a few months to get back into a natural sleep pattern. Go to bed when you are tired, and allow your body to wake you in the morning (no alarm clock allowed). You may find yourself catatonic (有紧张感的)in the beginning of the recovery cycle: Expect to have ten hours' shut-eye per night. As the days pass, however, the amount of sleeping time will gradually decrease.
So earn back that lost sleep —and follow the orders of your inner sleep needs, and you'll feel better. "When you put away sleep debt, you become superman." Says Stanford's Dement, talking about the improved mental and physical capabilities that come with being well rested.
72. The example of sleep math is used to show       .                                                                                          
A. in what case you build up a sleep debt
B. why you need six hours' sleep every night.
C. why you are full of energy even when lacking sleep.
D. you should drink coffee to keep energetic  
73. The author begins Paragraph 2 with       .
A. an example     B. a definition     C. an order     D. a story
74. By saying the underlined sentence in the last paragraph, Dement means       .
A. a superman always needs a lot of sleep
B. you can become superhuman after you repay your debt
C. you will be in a good state with enough sleep
D. you will become superhuman if you don't make up for sleep debt
75. What might be the most suitable title for the passage?
A. How can you keep energetic?     B. Can you have a good sleep?
C. What is sleep debt?             D. Can you catch up on sleep debt?